Noise is one of the main complaints from aircraft passengers. especially passengers on helicopter aircraft. Many attempts have recently been made to reduce noise in both the cabin and cockpit interiors. Some attempts have centered around adding absorptive materials around the cabin interior to prevent noise from the airframe from propagating into the cabin. Other attempts have been directed toward reducing vibration of various aircraft components since vibration is one of the primary causes of noise within the aircraft.
One source of interior noise is produced by the heating system. In particular, many conventional aircraft utilize a cabin heat system that is mounted outboard of the interior trim panels, or otherwise encased from the interior, and which channels bleed air from the compressor stage of the turbine engine. The bleed air is generally at about 450.degree. Fahrenheit and approximately 2-10 psi. This hot air is mixed in a mixing device with a flow of ambient air from the interior.
FIG. 1 is a graphical depiction of a prior art bleed air heat ejection cylinder which is commonly used to mix ambient air with hot bleed air. Air Comm of Boulder, Colorado manufactures one type of conventional bleed air heat ejection cylinder. The heater includes a cylindrical housing H which has an inlet end H.sub.l and an outlet end H.sub.o. Bleed air is fed to a perforated ring R that is mounted within the housing. The flow of bleed air draws ambient aircraft interior air into and through the housing, thus functioning as an air pump. The ambient air is drawn in through the inlet and passes through the ring. After the ambient air passes through the ring, it mixes with the hot bleed air. The combined flow is then channeled out of the housing and back into the cabin or cockpit. U.S. Pat. Nos. 4,978,064, 5,165,597, and 5,114,382 disclose some conventional bleed air heat ejection cylinder systems.
One of the primary deficiencies with this type of conventional system is that the inflowing hot bleed air tends to generate noise as it expands into the cylinder. The mixing action of the hot bleed air and cool ambient air flows also generates noise, similar to noise generated by the exhaust of a jet engine. The noise bounces off the walls of the housing and propagates into the aircraft interior.
The noise generated by heating systems in aircraft can be quite loud. reaching as high as 130 dBA, and results in passenger and aircrew discomfort. High noise levels also interfere with passenger and aircrew communication.
One attempt at solving this problem of heating system related noise has been by the incorporation of a uniform thickness foam liner F downstream from the perforated ring. While this solution has provided some noise reduction, the amount of noise generated by such a heat ejection cylinder is still substantial, particularly in the quieter executive configured aircraft interiors.
A need, therefore, exists for an improved heating system for aircraft that reduces the noise that occurs during mixing of hot bleed air with ambient cabin or cockpit air.